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Ok I got it now, I was using the Brush tool the last few days and somehow when I use the pencil tool, these white spots appear.
But is it normal, that these white spot appear when I use the pencil tool?
Its the same layer and sorry i dont know what an alpha channel is.

The pencil tool can act funky sometimes with complex brushes like these. So yeah, it's sorta normal but unfortunately I'll have to bow to someone who's more Gimp-knowledgeable for the why of it.

An alpha channel is the transparency of the layer. If any pixels are even partially transparent, you have an alpha channel in your layer. It's just like the R G and B channels for color, it just controls opacity.